The intestinal mucosal proteins which bind iron and other divalent metals that are absorbed in the duodenum will be isolated and characterized. These metals (Zn, Co, Mn, Pb, Ni) inhibit the absorption of each other and are absorbed in increased quantities in iron deficient animals. They seem to share binding sites on the same proteins. It is postulated that these proteins serve to regulate the absorption of these metals in the intestinal mucosa. Immunologic and biochemical methods will be developed for measurement of both the concentration of these proteins and their saturation with iron and other metals. These methods will be used to study intestinal specimens obtained from animals in the various physiologic states in which iron absorption is altered. In addition, the relative affinity of each divalent metal for the common metal binding proteins will be determined. It is believed that these data and knowledge of the various physiologic states in which the absorption of iron and the other divalent metals which share a binding protein will provide a basic understanding of the mucosal factors which regulate the absorption of iron. Although there are many similarities in the absorption of these metals, dissimilarities exist which may be explained by either differences in the binding affinity for a shared protein or the existence of unshared pathways or both. The dissimilarities will be exploited because it is believed that they will be helpful in explaining the specific role of each binding protein in different physiologic states that affect absorption.